How do I convert this FK leg to IK without getting rid of animation
You can't use an IK rig on Mixamo, you have to convert it at this stage.
This is the procedure:

In edit mode duplicate the foot bones (shift D), and put them in a different bone layer, for easier manipulation.

Clear their parenting (Alt P)

In pose mode set a copy transform constraint, in world space, targeting the respective Mixamo foot bone.

go to menu - Pose - Animation - Bake action, and check all preferences buttons

change the parenting of the toe bones from Mixamo feet to new ik feet

select the mesh and change the name of feet vertex groups from Mixamo names to new bones names

Delete the Mixamo feet bones

set an IK constraint on the calf bones, pointing at the Ik feet, setting the chain lenght to 2.


Now you have  working IK legs, without toe roll controls. To control the toe is a bit more complicated, but the basic concept is the same.